This invention relates generally to semiconductor manufacturing processes and particularly to the use of trench isolation in connection with semiconductor manufacturing processes.
In a variety of semiconductor devices, trench isolation is utilized to electrically isolate active areas from one another. Trench isolation is used in modem semiconductor processes to manufacture a variety of devices including transistors and photodiodes.
Photodiodes used in an imaging array can be manufactured using conventional complementary metal oxide semiconductor (CMOS) processing. As a result, an imaging array can be produced on a semiconductor structure which also contains logic circuits such as microprocessors, memories and the like. A pixel sensor uses pixels formed by photodiodes which receive light information from an image and convert the light information to electrical signals that are transferred to subsequent circuitry for further processing. The image sensors may be active pixel sensors or passive pixel sensors.
CMOS image sensors have advantages over the conventional charge coupled device (CCD) image sensors because they may achieve lower power consumption, integration of on-chip logic and lower cost. However when the CMOS image sensors are integrated with other logic circuits such as microprocessors, the sensors may need to be made with processes which include silicidation and trench isolation. Covering the photodiode with silicide would effectively block the light that is incident on the photodiode and thereby prevent the device from operating. However the silicide can be prevented from covering the photodiode through the addition of one or two masking steps.
The trenches used for trench isolation are generally dry etched and, as a result, the trench surfaces usually have a large number of interface states. These interface states lead to high surface generation velocity and, as discovered by the present inventors, a large dark current. The trench forming processes also cause crystalline defects such as dislocations and stacking faults. Those crystalline defects reduce carrier generation lifetime which increases dark current. Dark current is a current which flows when no light is incident on the camera. This is an undesirable consequence of trench isolation. The dark current decreases signal-to-noise ratios for the image sensor and decreases image quality.
Thus there is a continuing need for image sensors which can be manufactured with advanced logic processes to enable the image sensors and the logic devices to be integrated in a single integrated circuit. There is also a more general need for techniques for overcoming leakage currents in trench isolated semiconductor devices.